This is the second design (here is the first) of mine for a grow bed flooding sequencer. The aim here is to fill each grow bed in turn so as not to empty the fish tank and leave your fish walking more than they like.
The aim is to have it rotate from one tube to the next, stopping for long enough to fill the grow bed that's connected to each tube. The grow beds take it in turns to get filled, and as one is filling, another (or others) are emptying back into the fish tank.
When you first set it up, leave enough extra length on the downwards bit of the hooks so you can file a bit off to fine tune it to trigger at exactly the correct height. Or you could use a nut and bolt arrangement to make it adjustable.
Feel free to repost these images elsewhere, but please leave the "120 things in 20 years" caption on them.
[edit from the future - There is some additional material on sequencers. Readers might find this newer version in a post titled The Bullwinkle sequencer build of interest. It's a better design, and only costs around AU$15 to build with off the shelf PVC components]